The use of lost patterns consisting of films is known from German OS No. 1 926 163. These patterns can be produced in a simple manner by a known deep-drawing shaping process utilizing reduced or elevated pressure. The patterns are introduced into a casting box, whereupon the latter is filled with loose binderless sand. The sand is thereupon subjected to reduced pressure, so that the film of the pattern is pressed against the sand by the external air pressure. In this way it is possible to produce patterns of stable shape from very thin films. This stability of shape is however ensured in the case of thin films only as long as the reduced pressure is maintained in the sand. This method of casting has been found decidedly advantageous because the production of the castings entails only little expense, a casting having a very smooth surface is obtained, and the preparation of the sand is not necessary.
A prerequisite for the performance of the process described above is that the reduced pressure in the upper and lower parts of a two-part casting box should be maintained from the time when the pattern is completed until the casting has been completely cooled. In order to obtain a continuous work flow it is known to place the casting box on trucks of a closed-circuit rail conveyor. The length of the rail conveyor and the cycle time for the movement of the rail conveyor truck are so selected that after one circuit of the casting box the casting has cooled to such an extent that it can be removed from the casting box. If it is desired to achieve high cycle times, a long rail conveyor must be selected, because the cooling times are dictated. In long rail conveyors, however, the suction pipes connected to the upper and lower boxes are a distinct disadvantage. Known rail conveyors are circular in shape and have a central suction pipe from which flexible pipes lead to the individual casting boxes. These known rail conveyors make poor use of the available space, and in addition, because of the supply pipes, they cannot be indefinitely enlarged. Another disadvantage of these known rail conveyors is that manipulation of the casting boxes is limited by their continuous connection to the central suction pipe.
The problem underlying the invention consists in so constructing a casting box of the kind first described above that during the casting and cooling time the connection of the upper and lower boxes to flexible suction pipes is not necessary and that rail conveyors of any desired length can be used.
According to the invention this problem is solved through the fact that each of the evacuation pipes disposed in the upper and lower boxes can be connected to a suction pipe by way of a coupling closed by a valve, that the upper and lower boxes each have another coupling which is closed by a valve and by which a connection can be made between the suction pipe of the two boxes when the upper box is placed on the lower box, and that the lower box contains an additional coupling which is closed by a valve and by means of which the lower box, and consequently also the upper box placed on the lower box, can be connected to another suction pipe, which is for example installed in a rail conveyor truck.
The upper and lower boxes thus each have available a coupling to which suction pipes can be connected temporarily during the forming of the pattern. These couplings are necessary because the upper and lower boxes are made separately. When the upper and lower boxes are placed one on top of the other the evacuation pipes of the two boxes are connected together. It would at this point be conceivable that one of the two suction pipes could be removed. When the lower box is placed on the rail conveyor truck, it is connected by means of another coupling to the suction pipe of the rail conveyor truck. The lower and upper boxes are now evacuated with the aid of the suction pipe of the rail conveyor truck, whereupon the two suction pipes acting on the upper and lower boxes can be removed. Suction pipes leading to the upper and lower boxes are now no longer necessary. The suction pipes of the individual rail conveyor trucks can be connected together, so that rail conveyors of any length can be formed. The suction pipes of the rail conveyor trucks need then only be connected at one point to a central suction pipe. According to the invention each of the couplings of the casting box is provided with a nonreturn valve, so that on the removal of a suction pipe or on the lifting-off of the upper box or lower box the coupling freed is automatically closed.
In a preferred embodiment of the invention the coupling connecting the upper box to the lower box is in the form of a pin and a bore, the pin disposed on one box engaging a bore provided on the other box. Furthermore, according to the invention the coupling connecting the lower box to the rail conveyor truck is made in the form of a pin and a bore, the pin or bore being provided either on the lower box or on the rail conveyor truck.
In another advantageous embodiment, the bores are disposed in the lower box and the pins in the upper box and rail conveyor truck respectively. According to the invention, nonreturn valves consisting of a spring-loaded ball are provided in the pins. According to another proposal of the invention the sockets provided in the lower parts are disposed in line with and opposite one another and are closed by respective valve cones loaded by a common spring. This valve cone is provided on its end face with a push-rod which opens the nonreturn valve provided in the pin when the upper box is placed on the lower box or when the lower box is placed on the rail conveyor truck.